1. Field of the Invention
The present invention relates to a wavelength division multiplexing optical transmission system and an optical communication method, and more particularly to a wavelength division multiplexing optical transmission system compensating a dispersion at an transmission terminal.
All of patents, patent applications, patent publications, scientific articles and the like, which will hereinafter be cited or identified in the present application, will, hereby, be incorporated by references in their entirety in order to describe more fully the state of the art, to which the present invention pertains.
2. Description of the Related Art
In recent years, there have been aggressive researches and developments of technologies of long-distance optical transmissions at a high bit rate in the wavelength division multiplexing optical transmission system. Such technologies are disclosed, for example, (1) by K. Fukuchi et. al. in European Conference On Optical Communication, 1999, PD2-10, September 1999, entitled “1-Tb/s (55×20-Gb/s) dense WDM solution transmission over 3020 km widely-dispersion-managed transmission line employing 1.55/1.58-μm hybrid repeaters”, (2) by I. Morita et al. in Optical Fiber Communication Conference 2001, TuF5, March 2001, entitled “Benefit Of Raman Amplification In Ultra-Long-Distance 40 Gbit/s-based WDM transmission”, and (3) Y. Inada et al. in Electronic Information Communications Association, Communication Society B-10-78, September 2001 entitled “40 Gb/s EDM-2400 km Transmission Using Double-Hybrid Transmission Line”.
In order to realize a long-distance optical transmission at a high bit rate, it is important to reduce a dispersion slope of an optical fiber and namely reduce a wavelength-dependency of a dispersion appearing in the optical fiber. In the prior techniques disclosed in the above-mentioned literatures, a dispersion-flat transmission line is used, which utilizes a combination of a core-enlarged pure silica core fiber and a slope-compensated dispersion compensation fiber. In the dispersion-flat transmission line, a positive dispersion slope processed by the core-enlarged pure silica core fiber is compensated by a negative dispersion slope processed by the slope-compensated dispersion compensation fiber, whereby a total dispersion slope or an effective dispersion slope is reduced.
Even if the dispersion-flat transmission line is used, it is difficult for the present fiber-fabricating technique to reduce the dispersion slope into zero. In accordance with the first prior art disclosed in the above described first literature (1), an averaged dispersion value of the dispersion-flat transmission line takes a maximum in the vicinity of a center of a transmission band. Further, the dispersion-flat transmission line has such a wavelength-dependency of the dispersion value that the dispersion value becomes higher in a center wavelength band. The above described first literature (1) describes that the dispersion-flat transmission line exhibits a dispersion difference of about 150 ps/nm in long-distance optical transmissions of 3000 km between at a dispersion-maximum wavelength and at a dispersion-minimum wavelength in the transmission band. This dispersion difference is serious and large problem for the wavelength division multiplexing optical transmission system with a high bir rate of 40 Gb/s.
Transmission terminal separate dispersion-compensating methods as examples of the conventional techniques for compensating the dispersion slope of the transmission lines are disclosed in Japanese laid-open patent publications Nos. 62-18131 and 9-46318, wherein in a transmission station for multiplexing optical signals, respective optical signals are given different dispersions which depend on respective wavelengths prior to the multiplexing, whereby the dispersion slope of the transmission line is compensated. The dispersion compensation fiber is used for compensating the dispersion slope.
In accordance with the above transmission terminal separate dispersion-compensating methods, the same number dispersion apparatus as the number of channels to be multiplexed are needed. This results in an undesired enlargement of the apparatus of the transmission station and an undesired increase in the cost of the apparatus.
Further, the above transmission terminal separate dispersion-compensating methods are not practically applicable to the wavelength division multiplexing optical transmission system using a polarization interleaving multiplexing technique. In order to apply the polarization interleaving multiplexing technique, it is necessary that polarized waves remain conserved or preserved until the polarized waves are multiplexed. It is, however, very difficult for the present fiber-fabricating technique to prepare a polarization-conserving dispersion-compensating fiber which conserves the polarized waves until the polarized waves are multiplexed. For those reasons, it is thus difficult that the transmission terminal separate dispersion-compensating method compensating the dispersion by the dispersion-compensating fiber is used in conjunction with the polarization interleaving multiplexing technique.
Other dispersion-compensating techniques are disclosed in Japanese laid-open patent publications Nos. 2001-86065 and 2001-103006, wherein Faraday rotator mirror is used for a dispersion-compensating apparatus which compensates the dispersion with conserving the polarized waves. The use of the dispersion-compensating apparatus using the Faraday rotator mirror results in undesired size-enlargement and cost-increase of the apparatus.
In order to render available the desired utilization of the polarization interleaving multiplexing technique, sill another dispersion-compensating method is utilized in many optical transmission systems, wherein optical signals are multiplexed to generate a wavelength division multiplexing optical signal which is then entered into and transmitted through a dispersion compensation fiber for compensating the dispersion slope. In this case, the dispersion compensation fiber to be used for compensating the dispersion slope does not need to conserve the polarized waves. The prior arts disclosed in the above-described literatures (1), (2) and (3) also utilize this dispersion-compensating methods. It is practically difficult for the known dispersion compensation fiber to perform a desired optimum compensation to the dispersion of the optical signal in a full transmission-wavelength band.
A conventional wavelength-dispersion compensating device as yet another technique for compensating the dispersion slope of the transmission line is disclosed in Japanese laid-open patent publication No. 11-284263. This conventional wavelength-dispersion compensating device demultiplexes multiplexed optical signals by a wavelength division multiplexing filter, and compensates respective dispersions of respective demultiplexed-optical signals by a grating fiber, and further re-multiplexes the dispersion-compensated demultiplexed-optical signals. The last-mentioned Japanese publication No. 11-284263 discloses that the conventional wavelength-dispersion compensating device is inserted into the middle of the transmission line of the optical transmission system.
A conventional optical waveguide grating as further another technique for compensating the dispersion slope of the transmission line is disclosed in Japanese laid-open patent publication No. 2000-221338. This conventional optical waveguide grating is used as a dispersion-compensating device which performs simultaneous compensations of optical signals multiplexed. The conventional optical waveguide grating has a secondary-functionally variation of grating pitch along a longitudinal direction of the optical waveguide, so as to enable the conventional optical waveguide grating to perform simultaneous compensation of the dispersion and the dispersion slope. This Japanese publication No. 2000-221338 does not address where the conventional optical waveguide grating is used in the optical transmission system.
A furthermore conventional technique for simultaneously reducing both the dispersion and the dispersion slope is disclosed in Japanese laid-open patent publication No. 2001-197003, wherein a dispersion-compensating device includes a first compensating means having a wavelength-dependent variable wavelength-dispersion characteristic dependent upon wavelength for compensating a dispersion slope of an inputted optical signal and a second compensating means having a wavelength-independent constant wavelength-dispersion characteristic being constant independently from wavelength for compensating a dispersion of the inputted optical signal. A dispersion compensation fiber (DCF) is used as the first compensating means for compensating the dispersion slope of the inputted optical signal. A virtually imaged phased array compensator is used as the second compensating means for compensating the dispersion of the inputted optical signal. This Japanese publication No. 2001-197003 also discloses that this dispersion-compensating device may be placed at any position of the optical transmission system.
In the above circumstances, the development of novel wavelength division multiplexing optical transmission system and optical communication method free from the above problems were desirable.